There are more than 100 different types of cancers known in the art to date. It has been suggested that the vast catalogue of cancer cell genotypes is a manifestation of six essential alterations in cell physiology that collectively dictate malignant growth. One of these is the ability to evade apoptosis, a form of programmed cell death occurring in metazoans (Hanahan and Weinberg, 2000). This acquired resistance to apoptosis is a hallmark of most and perhaps all types of cancer. When components of the apoptotic machinery are altered, the dynamics of tumour progression are drastically altered. Resistance to apoptosis can be acquired by cancer cells through a variety of strategies, but the most commonly occurring loss of a pro-apoptotic regulator is through the mutation of the tumour suppressor genes. The resulting functional inactivation of their products (onco-proteins) is seen in greater than 50% of human cancers and results in the removal of a key component of the DNA damage sensor that can induce the apoptotic effector cascade. Most regulatory and effector components are present in redundant form. This redundancy holds important implications for the development of novel types of anti-tumour therapy, since tumour cells that have lost pro-apoptotic components are likely to retain other similar ones (Zhivotovsky and Kroemer, 2004).
Oesophageal cancer (OC) or the cancer of the oesophagus is one of the least studied and deadliest cancers worldwide. In 2008, it was estimated that approximately 488 084 new oesophageal cancer cases had emerged with a mortality of 412 628. In the year 2030, these numbers could double exponentially (Ferlay et al., 2010). It is caused by various factors but mainly by oxidative damage such as tobacco smoking, excessive alcohol consumption, human papillomaviruses (HPV), fungal mycotoxin contamination in food, nutritional deficiencies in vitamins such as zinc, nicotinic acid, magnesium, riboflavin, selenium, and a lack of fruit and vegetables in the diet. The incidences vary globally where it is the highest in the East-Africa, South-Africa, China and central Asia regions and the lowest in the Western Africa, South-East Asia, United States and South American regions (Garcia et al., 2007). It is more prominent in black men who have a higher incidence of the disease than men in other racial or ethnic groups. In South Africa, it is the second most common cancer among all South African men combined and the most common cancer in black males (especially those in the Transkei area). Epidemiologically, OC can occur in two different forms: the squamous cell carcinoma (SCC) and the oesophageal adenocarcinoma (AC). Squamous-cell carcinoma is caused mainly by factors such as the chronic irritation and inflammation of the oesophageal mucosa, excessive alcohol intake and smoking. Oesophageal adenocarcinoma is mainly caused by the existence of gastro-oesophageal reflux disease (GERD) defined by hiatal hernia, oesophageal ulcers and the presence of the Helicobacter pylori infection (strains for the CagA protein) in the gastrointestinal system (Bird-Lieberman and Fitzgerald, 2009; Enzinger et al., 2003; and McCabe et al., 2005).
Breast cancer is the most commonly diagnosed cancer and cause of death in women worldwide (Ferlay et al., 2008 and Garcia et al., 2007) and in 2007 there were 1,301,867 estimated new cases and 464,854 estimated deaths from breast cancer (Garcia et al., 2007). In South Africa, breast cancer is also the most commonly diagnosed cancer in women and it constitutes on average 20% of all cancers reported in South African females (Vorobiof et al., 2001). According to the National Cancer Registry of South Africa in 2001, breast cancer was the leading cancer in Coloured and Asian women with respectively 25.48% and 30.17% of all cancers reported being breast cancer. In black women, breast cancer is the second most commonly diagnosed at 17.25%, after cancer of the cervix and in white women at 18.41% second only to basal cell carcinoma.
Transformation of various epidermal cells can give rise to several types of skin neoplasms (abnormal tissue mass) that can be divided into two main categories, namely: 1) Non-Melanoma Skin Cancer (NMSC) predominantly arising from transformed keratinocytes and 2) Cutaneous Malignant Melanoma (CMM) predominantly arising from transformed melanocytes (Scherer and Kumar, 2010). Melanomas are usually heavily pigmented but can be amelanotic. CMM can be further sub-divided into four categories classified according to clinical and histological characteristics and they are: 1) Superficial Spreading Melanoma (SSM), 2) Nodular Melanoma (NM), 3) Lentigo Maligna Melanoma (LMM) and 4) Acral Lentiginous Melanoma (ALM) (Garbe et al., 2010).
CMM has been classified as a multifactorial disease where both environmental and genetic factors/mutations interact in concert to contribute to CMM susceptibility with CMM contributing to approximately 5% of all skin cancers and approximately 1% of all malignant tumours (Bressac-de-Paillerets et al., 2002). The World Health Organisation (WHO) estimated that 132 000 new cases of CMM arise per annum internationally with CMM incidence rates showing an increase of 28% in men and 12% increase in women worldwide. The highest CMM incidence rates worldwide are found in Australia and New Zealand where it has been classified as the third most common cancer in women and fourth in men. CMM has been reported to account for 90% of all deaths associated with cutaneous tumours (Garbe et al., 2010; Giblin and Thomas, 2007). Interestingly no papers on studies have been conducted on CMM incidence rates in South Africans nor is there a skin cancer registry currently present in South Africa; however, CANSA (Cancer Association of South Africa) has reported that skin cancer is the most common cancer in this country with an estimated 20 000 new cases being reported per annum.
Lung cancer is generally caused by smoking and in minor cases, exposure to carcinogens. In South Africa, lung cancer accounted for 67% deaths in men for 2006 while 32% in women. Of the newly diagnosed cases during 2008 worldwide, lung cancer contributed 13% (Ferlay, Shin et al., 2010; Bello, Fadahun et al., 2011).
In cancer, the normal mechanisms of cell cycle regulation are dysfunctional, oncogene and tumor-suppressor gene mutations drive the neoplastic process by increasing tumor cell number through over-proliferation of cells or the inhibition of cell death or cell-cycle arrest (King and Cidlowski, 1998; Vogelstein and Kinzler, 2004; and Lowe et al., 2004). The increase can be caused by activating genes that drive the cell cycle or by inhibiting normal apoptotic processes. In healthy breast cells, apoptosis occurs at varying rates during the estrus cycle in response to changes in hormone levels. Apoptosis is however also regulated by non-hormonal signals.
Apoptosis was first characterized in 1972 when Kerr et al. (1994) published their research recognizing apoptosis as an occurrence in adults relevant to health and disease (Kroemer et al., 2007). It is now defined as a discrete sequence of well coordinated and strictly controlled energy-requiring processes in which ligand binding to death receptors or cytotoxic insults result in the activation of several proteases and other hydrolytic enzymes (Bayir and Kagan, 2008). This cascade of cell signaling and Caspase-mediated events regulating proapoptotic and antiapoptotic proteins leads to proteolysis, rounding-up of the cell and shrinkage or reduction of cellular volume, chromatin condensation (pyknosis) and segregation along the nuclear membrane, classically little or no ultrastructural modifications of cytoplasmic organelles, nuclear fragmentation (karyorrhexis) and DNA fragmentation into mono- and oligonucleosomal units, plasma membrane blebbing—although the membrane is well maintained and does not release its contents or influence behavior of adjacent cells (Simstein et al., 2003; Holdenrieder and Stieber, 2004; Elmore, 2007; Golstein and Kroemer, 2007; and Kroemer et al., 2009). Resulting cell fragments, or apoptotic bodies, are phagocytosed by macrophages and surrounding cells. Loss of phospholipid asymmetry in the plasma membrane and phosphatidylserine externalization plays an important role in the opsonization of apoptotic bodies and subsequent phagocytosis (Zimmermann et al., 2001; Danial and Korsmeyer, 2004; and Holdenrieder and Stieber, 2004).
While apoptosis involves the activation of catabolic enzymes leading to the demolition of cellular structures and organelles, autophagy is a type of cell death in which parts of the cytoplasm are sequestered within double-membraned vacuoles and finally digested by lysosomal hydrolases. Autophagy is considered to be required for the starvation response and normal turnover of cellular components and in addition may be involved in a certain type of cell death (Mizushima, 2004; Festjens et al., 2006). Autophagy is characterized by the sequestration of cytoplasmic material within autophagosomes, which are double membrane structures containing undigested cytoplasmic material including organelles and cytosol, for bulk degradation by lysosomes. The fusion between autophagosomes and lysosomes generates autolysosomes, single membrane structures in which both the autophagosome inner membrane and its luminal content are degraded by acidic lysosomal hydrolases. Autophagosomes and autolysosomes are generalized as autophagic vacuoles and, unfortunately, it is sometimes difficult to distinguish autophagic vacuoles from other structures just by morphology. Morphologically, autophagic cell death is defined (especially by transmission electron microscopy) by massive autophagic vacuolization of the cytoplasm and the absence of chromatin condensation (Kroemer et al., 2009). The functional relationship between apoptosis and autophagy is complex, and autophagy may either contribute to cell death or constitute a cellular defense against acute stress, in particular induced by deprivation of nutrients or obligate growth factors (Kroemer et al., 2007).
Necrotic cell death or necrosis is morphologically characterized by a gain in cell volume (oncosis), distended endoplasmic reticulum; formation of cytoplasmic blebs; condensed, swollen or ruptured mitochondria; disaggregation and detachment of ribosomes; disrupted organelle membranes; swollen and ruptured lysosomes; plasma membrane rupture and subsequent loss of intracellular contents. Necrosis has been considered as an accidental, uncontrolled form of cell death (Zong and Thompson, 2006; Elmore, 2007; and Kroemer et al., 2007), but evidence is accumulating that the susceptibility to undergo necrosis is partially determined by the cell (and not only by the stimulus) and that the necrotic process involves an active contribution of cellular enzymes, implying that execution of necrotic cell death may be finely regulated by a set of signal transduction pathways and catabolic mechanisms. Necrotic cell death is not the result of one well-described signaling cascade but is the consequence of extensive crosstalk between several biochemical and molecular events at different cellular levels. Recent data indicate that the serine/threonine kinase, receptor interacting protein 1 (RIP1), which contains a death domain, may act as a central initiator (Golstein and Kroemer, 2007; and Kroemer et al., 2009). Calcium and reactive oxygen species (ROS) are main players during the propagation and execution phases of necrotic cell death, directly or indirectly provoking damage to proteins, lipids and DNA, which culminates in the disruption of organelle and cell integrity. Necrotic cell death is still largely identified in negative terms by the absence of apoptotic or autophagic markers, in particular when the cells undergo early plasma membrane permeabilization as compared with its delayed occurrence in late-stage apoptosis. There is however often a continuum of apoptosis and necrosis in response to a given death stimulus. Many insults induce apoptosis at lower doses and necrosis at higher doses. Necrosis is considered to be harmful because it is often associated with pathological cell loss and because of the ability of necrotic cells to promote local inflammation that may support tumor growth. Since apoptotic cells are rapidly phagocytosed prior to the release of intracellular contents without induction of an inflammatory response (Zimmermann et al., 2001; Holdenrieder and Stieber, 2004), apoptosis is usually the preferred method of cell death.
The central component of apoptosis is a proteolytic system involving a family of proteases called Caspases or cysteine aspartate-specific proteases (Zimmermann et al., 2001). Members of the mammalian Caspase family can be divided into three subgroups depending on inherent substrate specificity, domain composition or the presumed role they play in apoptosis. They include initiators in apoptosis, Caspase-2, 8, 9 and 10 executioners in apoptosis, Caspase-3, 6 and 7 and the Caspases that participate in cytokine maturation and inflammatory responses, Caspase-1, 4, 5, 13, 11, 12, and 14 (Zhang et al., 2004; Elmore, 2007; and Chowdhury et al., 2008).
Initiator Caspases are able to activate effector Caspases or amplify the Caspase cascades by increased activation of initiator Caspases. The effector Caspases then cleave intracellular substrates, culminating in cell death and the typical biochemical and morphological features of apoptosis (Zimmermann et al., 2001). They are all expressed as single-chain proenzymes (30-50 kDa) that contain three domains: an NH2-terminal domain, a large subunit (approx. 20 kDa) and a small subunit (approx. 10 kDa). Caspases are synthesized as inactive zymogens (proenzymes) which require a minimum of two cleavages, one separating the prodomain from the large subunit and another separating the large and small subunits, to convert it to the mature enzyme (Zimmermann et al., 2001; Zhang et al., 2004; Wang et al., 2005; and Chowdhury et al., 2008). All of these cleavages involve Asp-X bonds and the cleavage between the large and small subunits precedes removal of the prodomain. Eventually a heterodimeric enzyme is formed with both fragments contributing to the formation of the catalytic machinery. Initiator Caspases possess long N-terminal prodomains that contain recognizable protein-protein interaction motifs while effector Caspases have short or no prodomains. Usually, initiator Caspases, once activated, will activate the downstream effector Caspases in a cascade-like pattern. The effector Caspases then cleave their substrates which are usually either regulatory proteins that function in the homeostatic pathways or proteins involved in the organization and maintenance of cell structures. Caspases can modify the function of their target substrates either by inactivating their normal biochemical function, activating them by removal of regulatory domains, alter or invert their function or play a proteolytic role in the disassembly of the structural components of the cytoskeleton and nuclear scaffold. The cleavage of the proteins results ultimately in cellular, morphological and biochemical alterations characteristic of apoptosis (Zimmermann et al., 2001; Zhang et al., 2004; Wang et al., 2005; and Chowdhury et al., 2008).
Although morphologically similar, there are several distinct subtypes of apoptosis that can be triggered through several molecular pathways. The two major biochemical pathways are the extrinsic or death receptor pathway and the intrinsic or mitochondrial pathway (Zimmermann et al., 2001). In the extrinsic pathway, apoptosis is activated through ligation of extrinsic signals to ‘death receptors’ (DR) at the cell surface which triggers intracellular signaling that leads to Caspase-8 activation (Zimmermann et al., 2001). The intrinsic pathway involves pro-apoptotic signals that translocate to the mitochondria, resulting in mitochondrial membrane permeabilization (Zimmermann et al., 2001). This, in turn, provides a route for the release of intermembrane space proteins such as cytochrome C into the cytosol which promotes the formation of the apoptosome, a molecular platform for the activation of Caspase-9. There are additional apoptotic pathways, the granzyme A-mediated pathway, the granzyme B-mediated pathway and the endoplasmic reticulum (ER)-mediated pathway.
Following initial induction, the extrinsic, granzyme B- and ER-mediated pathways merge at the level of the effector Caspases which cleave specific cellular substrates such as regulatory enzymes that result in the processing of intracellular structural proteins and eventually leads to apoptotic cell death.
The term “chemotherapy” was first introduced by Paul Ehrilich who defined it as the systematic treatment of both infectious disease and neoplasia. Chemotherapy began in 1942, when it was discovered that nitrogen mustard was an effective treatment for cancer. Cancer chemotherapy aims to treat cancer with chemicals that maximize the killing of neoplastic cells while minimizing the killing of most/all other host cells (Chabner and Roberts, 2005 and Pitot, 2002). Typical chemotherapeutic agents used today include dexamethasone, Cisplatin, etoposide, cytosine arabinoside, taxol, 5-fluorouracil, doxorubicin, topotecan and bleomycin. Most chemotherapeutic agents induce programmed cell death/apoptosis via the endogenous or intrinsic mitochondrial pathway. Chemotherapy is able to induce apoptosis in four stages (Bold et al., 1997), namely:    (i) Chemotherapeutic agent disrupts cellular homeostasis through a specific interaction with an intracellular target;    (ii) Recognition by the cell of the disruption of homeostasis;    (iii) Cell deciphers the severity of the injury and decides either to repair the injury or to proceed with apoptotic cell death; and    (iv) Initiation of apoptosis leading to cell death.
In recent years, a re-examination of the knowledge of chemotherapeutic-induced cell death has been revisited due to the complexity of cell death mechanisms (e.g. autophagy, senescence). The idea is to perhaps precisely activate or inhibit molecules that mediate the diverse forms of cell death, aiming ultimately to develop less toxic and more effective chemotherapeutic regimens.
Certainly most current therapies are of a ‘blunderbuss’ nature (Cotter, 2009), and the designing of target-specific chemotherapeutic drugs are desirable.
Tertiary phosphine complexes of silver(I) were first prepared in 1937 (Mann et al., 1937). These coordination complexes of silver(I) salts display a rich diversity of structural types. The potential application as anti-cancer drugs of silver(I) salts follows from the analogy of the chemistry of silver(I) salts with that of gold(I) salts. Notably, however, literature on silver(I) complexes as potential anti-cancer drugs is limited to a small number of analogues of the so-called cationic lipophylic complexes. These complexes typically consist of a coinage metal (gold, silver, copper) chelated between bidentate phosphines causing a tetrahedral environment. Silver(I) complexes of bidentate pyridylphosphines were of particular interest to Meijboom et al., 2009 in their context as potential antitumour agents (Berners-Price, 1998 and Liu, 2008). The interaction between bidentate diphosphines R2P(R′)nPR2 and silver salts has attracted a great deal of interest because the resultant complexes have found some application in homogeneous catalysis (Sawamura, 1990) and also as antitumour compounds (Berners-Price, 1988 and Barnard, 2007). However, the interplay of various parameters such as the geometrical flexibility of Ag(I), the bite angle and the electronic properties of the phosphine as well as the coordination mode of the supporting ligands, often renders predictions concerning the structural properties of silver-phosphine complexes difficult (Berners-Price, 1988), both in solution and in the solid state (Brandys, 2002). This difficulty is exemplified by the crystallisation of these complexes in more than one polymorph. Generally, a tetrahedral environment around the silver(I) atom is preferred in the solid state, however many exceptions to this tetrahedral environment have been observed. To date, little systematic work has been reported on the interaction between silver salts and diphosphines characterized by different spacers (R′) or substituents R. Despite the obviously interesting structural chemistry of silver(I), and the potential of these complexes as anti-cancer agents, cognizance should be had to the fact that relatively few publications on silver(I) complexes as anti-cancer agents have been reported in literature.
While little systematic work is reported on the interaction of diphosphines with silver, and their resulting applications as anti-cancer agents, even less has been reported on the use of monophosphines. To date, the only systematic studies on monophosphines with silver are two series of papers by White et al. on the coordination ability of PPh3 and PCy3 to silver(I). These papers (reviewed in Meijboom, 2009) dealt exclusively with the structural chemistry of the resulting complexes and did not report any (potential) applications. What became clear from these papers, however, is that the coordination chemistry of silver(I) complexes is an extremely rich and varied field, with a range of structural types (nineteen different structural types have been indentified in Meijboom, 2009). A comprehensive (and ongoing) search of the literature revealed only two papers by Zartilas et al. (2009) on the coordination of P(4-MeC6H4)3 to silver halides reporting extremely limited data on anti-cancer testing (IC50 values only).
In the light of the foregoing, it is apparent that there is a clear need in the art to explore the potential of silver(I) monophosphine complexes as anti-cancer agents with a view to understanding the molecular mechanisms through which these complexes induce cell death and the mechanism of cytotoxicity of these complexes.